Trunked radio networks are typically used by a closed user group, such as company employees, the police, the fire brigade, other public safety authorities or military forces. The early trunked radio networks were completely closed without any interconnection to other communications networks. Their main use was traditionally to share information between a central node and the personnel carrying radio terminals.
Inter-network calls from a trunked radio network to another network are, however, desirable since a user carrying a trunked radio terminal for the particular trunked radio network may need to be in contact with persons on other trunked radio networks or public communication networks not being connected to the particular trunked radio network. Without any inter-connection between the particular trunked radio network and other networks, the user would need to carry separate terminals, one for each network.
A known solution for inter-connection between networks uses a gateway that can be addressed from one network to route connections to another network connected to the gateway. A telephone call made by a user is first routed to the gateway and there the routing is carried out to establish a link to the recipient of the telephone call. The user first dials a gateway number to establish a connection to a specific gateway relaying the used trunked radio network to a destination network and then gives an intended target telephone number. This, however, is not seen as a satisfactory solution, because a typical user does not want to remember both the telephone number of an intended recipient and the gateway number related to that telephone number.
A solution to the above-mentioned problem is given in WO98/20662. This document discloses a calling subscriber station having a correspondence table storing the numbers of necessary gateways. When a telephone number is dialled, the calling subscriber station associates a corresponding gateway number on the basis of analysis of, for example, the leading digit of the dialled telephone number.
However, the numbering schemes of different networks are not always consistent and this solution is system dependent. For this reason, this document proposes adding a separate character, for example, a prefix or a suffix, which will then be recognised by the calling subscriber station to associate the telephone number with the corresponding gateway. This approach is also unsatisfactory, since the user of the calling subscriber station would have to know which character to add and where to place it in the dialling sequence when calling a subscriber in another network.
Furthermore, a user of a trunked radio network communications terminal is likely also to be using a cellular mobile station in connection to a Public Land Mobile Network (PLMN). Whether the used cellular mobile station is in private or business use, the user is likely to get used to the functioning of its user interface. Therefore, the need to remember different prefixes for different networks for trunked radio network use is cumbersome.